Elevated Exhaust Back Pressure

[76oDay]

(Taken from Moyer Marine Spring Newsletter)

"The Breath of Life"
It's obviously difficult to know for sure, but given the number of engine issues showing up in recent years resulting from confirmed excessive exhaust back pressure, we believe that at least half of the Atomic 4 fleet may be suffering from some elevation of exhaust back pressure over the factory recommended range of 1 to 1.5 psi.

Many of the early symptoms of elevated exhaust back pressure (3 to 4 psi) mimic other conditions such as a rich fuel condition, low compression, poor ignition, or all of the above. Specific symptoms include uniformly sooty plugs, carbon build-up on valves and rings, caramelized goo on intake valve stems, and in worst cases (usually when back pressure exceeds 6 psi), one or two cylinders will refuse to fire.

Two most common causes of elevated back pressure are mineral build-up in the hot section where the engine cooling water enters (see photo below), and the inner lining in the first several feet of the rubber hose on the discharge end of water lift mufflers breaking away from the rest of the hose and crumpling up on itself.



Measuring the back pressure at the exhaust manifold flange is the direct manner of determining whether or not you have a problem. Otherwise, if you have the symptoms and you've insured that the other causes are not present, it's probably time to remove your hot section for a look (the hard pipe between the manifold and the inlet to the muffler).

What may be new to many of you is the fact that when an engine is struggling for intake air, it will develop many of the same symptoms as restricted exhaust. In both cases, the smooth flow of air and exhaust through the engine is restricted. A very interesting recent case involving symptoms of elevated exhaust pressure (including sticky intake valves) turned out to be the result of setting a briefcase in the lazarette in such a manner as to block the 6" diameter vent between the engine compartment and lazarette. The lesson from this episode is simple: if your engine compartment is well sealed, be sure that the vent for the engine air supply is adequate and unrestricted. We don't have a lot of data regarding an exact vent size, but a 6" diameter vent appears to be adequate. Obviously, a severely clogged flame arrester element can cause the same "breathing" issue for an engine.

Hi,

This is GREAT info.! This is exactly what I was experiencing as a
significant problem last year. With the help of some fellow sailors at my
marina, this is what we came up with also. Initially I didn't appear to be
exhausting enough water, the exhaust water was sooty, steamy, and engine
temp. was elevated/overheating at higher RPMS. and workload. I'm pretty sure
that engine performance wasn't "up to snuff" either (only my 2nd full season
& most of the time was spent under sail previously). Since the boat was
already in the water I just "took it easy" for the season. At the end of the
season, when the boat was out of the water, I took apart the exhaust, at the
manifold through the last rubber hose at the boat aft. The first hose going
into the muffler was just like you describe, crumpling in on itself, while
the rest of the exhaust system appeared to be absolutely clear. My cast iron
exhaust pipes were not clogged, but have worn a little thin, so I went to
the local plumbing supply store in order to find replacements. I'm looking
forward to the exhaust rebuild as being the fix for this season. Thanks for
reinforcing my troubleshooting conclusions.

Pete

Congratulations Pete! Yours was a really classic case of elevated exhaust
back pressure and you did a great job of troubleshooting and remediation.
It would be wonderful if you could come up with a few photos of the inside
of your rubber hose with the crumpled up inner layer and post them in the
exhaust section of the Community Forum at moyermarine.com (or just a text
only description if you already discarded the hose). It's difficult for
anyone who hasn't experienced such an episode to clearly explain how it
feels.

Thanks again for your feedback,

Don Moyer

See my attached photos & Good Luck to all other sailors!

Pete

[hanleyclifford]

exhaust system back pressure

76oDay - I would be interested to see a picture or just a layout of your exhaust system. Most of the systems I have seen on this scene have been complicated and contorted, usually because of the requirements imposed by the available space in the boat. The two most important factors affecting back pressure in an exhaust system are 1) number and severity of bends and 2) cross sectional area or ID of the components. The most restrictive component will govern. Just something to think about; increasing pipe size from 1 1/4" to 1 1/2" ID raises crossectional area more than 40% - almost half again.

[Qben]

"The most restrictive component will govern."

I think this statement needs clarification. Having a 1 1/4" ID section in an exhaust system that is otherwise 1 1/2" ID would not have as signifigant of an effect as if the whole system were 1 1/4"

I hope that clarifies more than it confuses.

Qben Oliver

[hanleyclifford]

further clarification

I don't know where you got the idea of 1" section in the system. The idea is to increase the size of the system, not reduce it. The smart play on these engines is to open up the exhaust flange to 1 1/2" using a NPT tap (the flange comes from the factory at 1 1/4" NPT. You will be stunned at the increase in power.

[Qben]

The 1 inch section was just an example. Certainly not a suggestion. I guess what I was trying to clarify was that it is beneficial to open up whatever portion of the exhaust that you can get to, even if you can't open up the whole thing. Your statement might lead people to believe that if they have a 1 1/4" part of the exhaust (flange for example) that they can't or don't want to increase to 1 1/2", that they might as well forget about making anything bigger since the "most restrictive component will govern."

I don't think this is the case at all.

You do bring up a good point though. I'll edit the previous post so I don't inadvertently lead someone to believe that putting a 1" ID section in the exhaust would be ok.

Qben Oliver

[hanleyclifford]

exhaust system pressure

Certainly there is always a benefit to making any component of a system better, however, the most restrictive component will remain the overriding consideration, and any changes of other components will obtain much lesser benefits. That is why the entire system should be scrutinized to see where the greatest benefit can be had. this means going to the "MRC".

[hanleyclifford]

your recent edit

I have to agree with you 100%, or to put it another way; making your exhaust run all the way down a 1 1/4" hole is much worse than squeezing it thru a short 1 1/4" section, and then giving it 1 1/2" to romp the rest of the distance!

[Qben]

That's what I was getting at. Obviously, I could have been more clear.

[Qben]

Pictures

I haven't seen any really good pictures of the telltale "caramelized goo" that Don says is indicative of elevated exhaust back pressure, so I thought I would add some for everyone's viewing enjoyment. These came off the engine I just bought as a spare. I think it might have had some back pressure issues!

Intake port on manifold side of block. The other one looked the same.


Three of the four intake valve holes.



This is the intake valve closest to the exhaust flange. It was really stuck in there. This stuff is hard to describe. It won't wipe off (without a solvent of some kind), and it's sticky like tree sap, but way thicker.


Qben Oliver

[keelcooler]

Our motors can take the heat, the flex exhaust hoses can’t. It does not take long to blister the inner walls just below the mixer during a serious blockage or no water event. Ever forget to open the raw water intake? Looks like a good reason to consider metal exhaust pipe extension to the water trap.